17 



ring ' lias been given by various authors. It depends 

 into the body-cavity from a point just below the circum- 

 oral water vessel (PI. TV., fig, 47; PI. Y., fig. 52, c.oM.r.), 

 and appears to attain its greatest development in the inter- 

 radii, especially the anal and right posterior ones. Its 

 wall consists of a thin sheet of connective tissue containing 

 rounded nuclei, and its form in sections depends largely 

 upon the quantity of coagulable fluid contained at the 

 moment of fixation. Generally speaking, however, it may 

 be said to expand gradually and irregularly from its point 

 of attachment to the wall of the oesophagus to its 

 periphery. Here it becomes much more sacculated ; and 

 the nuclei are not only more numerous but assume in 

 some parts a definite arrangement. 



At one point in its circumference the lacunar walls 

 become thickened, very irregular, and confluent, so as to 

 form a roughly lenticular mass of connective tissue 

 traversed by numerous interlacing tubules. This is the 

 ' spongy organ ' (PL Y., fig. 51). Its tubules are in direct 

 communication with a number of thick-walled trunks 

 (PI. lY., fig. 45), some of which run directly from the 

 spongy organ to the oral end of the axial organ, and, 

 branching and anastomosing, form a sort of vascular 

 envelope thereon (PI. Y., fig. 52). Others run on along- 

 side the axial organ towards the aboral face of the visceral 

 mass, and there branch and assume more irregular forms 

 upon the internal border of the intestinal coil. Other 

 similar trunks issue from the blood-vascular ring to be 

 distributed around the intestine and in the body cavity, 

 and others to ramify in the substance of the tegmen 

 calycis. Lastly, there is an annular lacuna which lies 

 outside the periphery of the blood-vascular ring and is in 

 communication therewith (PL Y., fig. 52, gen. hi. Ic). 

 From it a thin-walled tubular branch arises in each radius 



c 



